An absolute encoder is a sensor for determining absolute positions of an object. The location of sensor and object in relation to one another is changeable. The sensor can register linear changes in location of the object and it can register rotating angular changes of the object. Sensors operating on contactless principles which determine the position of the object according to the optical or magnetic active principle are known. To this end, a sensor registers absolute coding of a code track and an evaluation unit evaluates the registered absolute coding and establishes the object position therefrom. Within the meaning of the invention, the absolute coding is a spatially resolved position specification.
Absolute encoders are used in multifaceted ways. In plant construction and engineering, they determine the positions of machine elements such as drives, swivel heads, rotary tables, etc. in relation to a reference system. In geodetic instruments such as theodolites, tachymeters, laser scanners, etc., they measure horizontal angles and vertical angles in relation to distant objects. In coordinate measuring machines, they register spatial alignments of robot arms, steering wheels, etc.
In the following, the special case of an optical code track is considered. An optical code track has a mechanical support in the form of a disk, a ribbon, etc. In this respect, FIG. 1 shows an example from the prior art according to EP1890113A1. Many adjacent code elements are arranged on the mechanical support of the optical code track, which code elements embody the absolute coding 10. Within the meaning of the invention, the code elements arranged in the track direction update the bijective position specification of the absolute coding from one code element to the next adjacent code element in a spatially resolved manner.
As a result of the presence of defined code elements, which respectively embody a discrete defined element of the code (and are then also considered code element by code element during the evaluation, wherein a state/value is established for each code element), it is possible to speak of a “digital” code here (in contrast to a continuous code, e.g. updated between 0 and 1, wherein any arbitrary intermediate value can be decoded into the respectively sought-after value such as the sought-after location specification on the basis of a defined conversion function; in this case, this is usually referred to as an “analog” code).
The code elements are e.g. light-transmissive rectangles which are arranged in an optically opaque residual region. The optical code track 1 is illuminated by light from a light source by way of the transmitted light principle. The code elements modulate the light. Light passed by the light-transmissive rectangles is registered by a sensor along the track direction; light not passed by the optically opaque residual regions is not registered by the sensor. The light-transmissive rectangles are imaged on the sensor as a cast shadow. The sensor generates state signals for registered light. In the case of relative motion of the optical code track and sensor, the sensor registers the absolute coding as a temporally discrete sequence of discontinuous bright/dark transitions.
The absolute coding has either a bijective position specification or a bijective code. Hence, the position specification is either established directly from the state signals or a position specification is assigned to the code of the state signal by way of look up in a table. Since the code elements and the sensor have a spatial extent, it is moreover possible to establish a centroid of the state signal in order to relate the established position specification to the centroid of the code element with sub-code element accuracy. Within the meaning of the invention, the width of the state signal in the track direction is referred to as signal width and the width of the sensor in the track direction is referred to as sensor width. A centroid of the code element is deduced from the centroid of the state signal. Moreover, the distance to a reference position in the track direction is determined from the centroid of the code element. Hence the state signal not only supplies a bijective position specification but also enables determination of the location of the code element in relation to a reference position.
However, as an alternative to determining the centroid, a person skilled in the art is also aware of different processes by means of which the precise position of the code element can be established on the basis of the registered code projection.
This is all carried out in order to determine the position of the object with high accuracy. Thus, positions of machine elements are determined with an accuracy of 1 μm and theodolites measure horizontal angles and vertical angles to objects at a distance of several hundred meters with an accuracy of 0.1 mgon. In order to be able to achieve such a high accuracy, systematic and non-systematic errors must be eliminated when determining the position of the object.
Highly accurate absolute encoders therefore comprise a plurality of sensors which are arranged with a fixed spatial relationship to one another and which redundantly register the absolute coding of the code track. By forming averages of the absolute coding registered redundantly it is possible to eliminate non-systematic errors when determining the position of the object.
The remaining systematic errors when determining the position of the object often have a harmonic nature. Such harmonic errors have multifaceted causes. Thus, they can be due to irregularly arranged code elements on the code track or be caused by thermal expansion of the code track, eccentricity of the mechanical support of the code track, mounting play of the absolute encoder, diffraction phenomena on code elements, etc. Moreover, the fixed spatial relationship between the sensors themselves and the regular arrangement of the code elements on the code track constitute periodic structures. The superposition of the periodic structures may form interfering moiré patterns in the case of optical absolute encoders. Moreover, according to the Nyquist-Shannon sampling theorem, information losses may occur when registering the absolute coding in the case where a selected sampling frequency of the absolute encoder is too small in relation to the maximum frequency of the code elements.
In this respect, WO2011/064317A1 describes a method for establishing error coefficients and a method for correcting the measured value of an absolute encoder using these error coefficients. The absolute encoder has at least two sensors and an optical code track. The sensors and the optical code track are movable relative to one another. The sensors register the absolute coding of the optical code track as a sequence of bright/dark transitions at different angular positions. The sensors are spaced apart from one another at an angle of at least 50 degrees. An evaluation unit establishes angle position values from the absolute coding registered by the sensors. By comparing the difference in angle position values of the sensors for a plurality of different angular positions, harmonic angular errors are represented as error coefficients in a Fourier series expansion. The angle position values are corrected by these harmonic angle errors.